My 2 cents on this. I play BI H/M, so naval invasions happen frequently. While blockading ports does not harm ai financially, blockading their naval ports can prevent future invasions. And yes, chances are they already have many ships abound, destroy them ALL and continue blockading the naval ports. They can still build ships but they will be in port and only get out if they can beat you. So use good ships to blockade naval ports.